Radio communications links serve to receive, amplify and "repeat" or transmit "wirelessly" through the air modulated radio and microwave frequency, RF, signals originating from a remotely located transmitter to the next distant RF "repeater" in the link or, alternatively, to the end of the link, the intended distant RF receiver, the latter of which uses the signal or processes it in known ways for conventional purposes. These wireless RF links serve as the backbone of modern radio, television, data and telephone communication. The geosynchronized orbiting satellite is one known vehicle used to carry RF communications repeater equipment, including the component amplifiers and antennas, and the airborne equipment is used as a communications link between a transmitter located at one location on the earth and a receiver located at another location on the earth and, vice-versa.
In a geosynchronous orbit the satellite as viewed from the earth appears stationary or fixed in position. Hence a directional antenna in a ground transmitting station, such as a parabolic antenna or phased array antenna, is aimed at the satellite, and the antenna radiates RF energy of a given frequency supplied to the feed by a transmitter. That RF energy is received at the satellites receiving antenna, at a reduced power level, due to losses occurring in the passage through space and through medium attenuation. The associated amplifiers carried in the satellite amplify the received signal and typically convert it to a slightly different frequency. This signal is them coupled to a transmitting antenna from which it is radiated into space toward the earth, the communications "downlink". The transmitting or sending antenna is a directional antenna, one which concentrates or focuses the energy to a limited area, and is aimed at an earth ground station. As examples antennas such as parabolic reflector, lens, phased array, or equivalent aperture antenna, all of which are known, are useful in that application. The RF energy accordingly is propagated through space to the ground station: or, more accurately, to an area or "footprint" on the earth containing the ground station.
Although a single RF signal was described in the preceding instruction, more typically a number of RF signals of different frequencies, forming different channels or carriers, are simultaneously transmitted, "multiplexed", and are simultaneously received so that larger amounts of information may be handled by the communications link in a given period of time. The merits of such satellite communication, which provide a kind of direct line of sight communication, over prior methods and technology includes lower cost and more effective communication. Those advantages are known and need not be considered further in this brief introduction to the background to the present invention. The limitations in that existing system is a greater interest. The present invention minimizes such limitations to enhance the reliability and efficiency of that communications link.
In most downlink communications systems multiple carrier frequencies are amplified separately by individual high power amplifiers such as traveling wave tube amplifiers or high power solid state amplifiers ("HPSSA") which combine in a power combiner the output of several solid state devices. To date, the traveling wave tube amplifier or "TWTA" as more often labeled has been the amplifier of choice in this application. The TWTA is an amplifier which contains a vacuum tube known as a traveling wave tube. This is a unique microwave vacuum tube device which relies upon the phenomena of slowing down a propagating RF signal inputted to the device and applied to an internal "slow wave" circuit structure by means of which the propagating RF signal extracts energy and is amplified in an "electronic interaction" process from electrons moving in the vacuum envelope between a cathode and anode under an electrostatic acceleration force created by a high dc voltage applied between the anode and cathode elements. The amplified RF signal continues along the slow wave circuit and is output from the TWTA. The reader is referred to the technical literature for additional details of this amplifier device.
TWTAs presently used in this application provide practical conversion efficiencies from the direct current, dc; that is the power extracted from the dc power supply and consumed, to RF output power that is on the order of twenty five per cent or larger at frequencies of twenty GHz in wideband operation; that is operation in which the TWTA is used to amplify a single signal contained within a wide bandwidth frequency range, one generally regarded as greater than 10% bandwidth by those skilled in the art. In order to maintain high power amplifier efficiency, a separate HPA is required for each signal to minimize generation of interference products between the signals. The overall efficiency referenced to the antenna, however, is degraded by losses in the transmission path required between the output of the TWTA and the antenna feed, those occurring from the necessary inclusion in that RF path in practical systems of redundancy switches, those which serve to provide reliable operation in the event of a failure of a HPA and multiple signal combining mutiplexer and diplexer filter losses, and long waveguide runs to the antenna feed. The same factors also apply to high power solid state amplifiers.
Another possible method seldom used employs a single very high power TWTA to amplify multiple RF carrier signals. This alternative requires the TWTA's output power to be purposely lowered in order that the amplifier operates in a quasi-linear mode to maintain between the several RF carriers being amplified a low intermodulation interference, the undesirable distortion causing transfer of some portion of one signal of one channel to a different signal in another channel. As a consequence the overall efficiency in this arrangement is reduced by a factor of two as compared to the system aforedescribed using a single TWTA for each carrier frequency.
A significant practical factor that impacts efficiency in those TWTA systems is bad weather. Bad weather interferes with RF transmission. It is vital to maintain reliable communications to the ground station despite the existence of rain or snow at that receiving site. To handle that situation the communication system designers apply a "rain margin" into the transmitter's design, sizing the transmitter power depending on frequency and permitted bad weather outage by at least ten dB above that power necessary for reliable communications needed in clear weather. TWTAs for this application are thus sized to provide a power output that is at least ten times larger, ie. 10 dB, than clear weather requirements. Consequently the TWTA consumes ten times the dc power consumption as would be consumed if the system were designed for good weather operation only. This necessary design is inherently inefficient.
Attempts to mitigate the inefficiency in the high power single TWTA approach by technical gimmicks or designs to make the output power "programmable" result in increased expense and complexity, questionable reliability and overall lower TWTA operational efficiency, although achieving some savings in dc power consumption at lower power levels.
A principal object of the invention is to increase the electrical efficiency of communications systems down-link RF amplifiers. The present invention provides a new amplifier architecture that eliminates the need for traveling wave tube amplifiers or single HPA HPSSA and the attendant system inefficiency. The invention introduces very high solid state semiconductor reliability and flexibility and efficiency not heretofore possible as a practical matter with high power amplifier systems. The overall dc conversion efficiency referenced to the antenna feed of the amplifier system is increased over that available with TWTAs despite the fact that the efficiency of the individual solid state amplifiers employed as part of the system is less than that of an individual TWTA.